CMake 3.17 deletes Qt translations when updating the TS files

With CMake 3.16.8 or less, the following workflow was working, but 3.17+ deletes the Qt's TS files. The idea is taken from Professional CMake, 7th Edition by Craig Scott

Requirements

cmake 3.17+, Qt 5.9+, ninja

Concept

To update the translation files for Qt, UPDATE_TRANSLATIONS can be enabled. When finished, disable it. CMake 3.17+ deletes the updated TS files.

Adjust paths in the script update_translation.sh to automate the workflow.

Steps to reproduce

git clone https://github.com/Macintron/QtTranslationDemo.git # or create files from below in QtTranslationDemo.
mkdir build  # NOT in QtTranslationDemo
cd build
cmake -G Ninja -DCMAKE_PREFIX_PATH=[pathToQtDir/5.15.1/clang_64] ../QtTranslationDemo
ninja
./demo

Output should be:

    Demo
    de.qm: 85 bytes
      Language: 'Deutsch'
      greeting: ''
    en.qm: 85 bytes
      Language: 'English'
      greeting: ''

cmake -DUPDATE_TRANSLATIONS=ON ../QtTranslationDemo
ninja demoTranslations
cmake -DUPDATE_TRANSLATIONS=OFF ../QtTranslationDemo
# new strings ("greeting") should haven been added to the ts files, but TS files are deleted instead!

Why are the TS files get deleted? Is this a bug in CMake 3.17+ or a bug in my workflow?

build info

The build directory must NOT be inside the sources.